Re: Need advice: x86 redudant compare to zero

[Dale Johannesen <dalej at apple dot com>]

> My question is: where and how would you suggest we do this 
> optimization. With peephole2? Or in combine? In i386.md, I see pattern 
> *subsi_2 looks like what I'd like to combine these two insn into:
>
> (define_insn "*subsi_2"
>   [(set (reg FLAGS_REG)
>     (compare
>       (minus:SI (match_operand:SI 1 "nonimmediate_operand" "0,0")
>             (match_operand:SI 2 "general_operand" "ri,rm"))
>       (const_int 0)))
>    (set (match_operand:SI 0 "nonimmediate_operand" "=rm,r")
>     (minus:SI (match_dup 1) (match_dup 2)))]
>   "ix86_match_ccmode (insn, CCGOCmode)
>    && ix86_binary_operator_ok (MINUS, SImode, operands)"
>   "sub{l}\t{%2, %0|%0, %2}"
>   [(set_attr "type" "alu")
>    (set_attr "mode" "SI")])

That's quite similar to several PPC patterns for andi. , and they work.
If you've got two other insns that look like the set's I'd expect 
combine
to merge them, and would look to see why it doesn't.